1. Field of the Invention
This invention relates to transparency viewing systems. In a primary application the invention relates to multiple-pass illumination of radiographic transparencies which are difficult to interpret because of inadequate contrast using single-pass illumination.
2. Description of Prior Art
Photographic transparencies often have over or under-exposed regions whose detail contrast is severely reduced. This is particularly true of radiographic transparencies which are under-exposed in relatively opaque regions of the body. Thus relatively subtle lesions, such as small tumors, cannot be visualized because of inadequate contrast.
One of the earliest examples of contrast enhancement is U.S. Pat. No. 2,152,353 issued to Hans Lewin on Mar. 28, 1939, entitled, "Roentgen Photography." In this patent a doubling of the contrast is achieved by illuminating the radiographic transparency with ultraviolet light and using a phosphorescent backing. This effectively doubles the contrast since the film density attenuates both the ultraviolet on the first pass and the visible fluorescent light on the second pass. Although this system inproves the contrast of low-density under-exposed regions, it significantly reduces the visibility of medium and high density regions. In these cases the doubling of the density makes these regions essentially invisible. Thus an additional conventional viewer with back-illumination only would be required. The radiologist would have to move the film from one viewing device to the other to view it over its entire range.
This problem was solved in U.S. patent application Ser. No. 661,959 filed 2/27/76, "Transparency Contrast Enhancement System" invented by Albert Macovski, and now Pat. No. 4,002,914. In this system a combination of front and back illumination is used to provide the desired visibility and contrast at all density ranges. In regions of very low density, the front illumination is used to double the contrast. In regions of relatively high density a source of back illumination is used to provide the required visibility.
The problem with each of these contrast enhancement methods, however, is the requirement for a source of front illumination. Since this light source must be on the same side as the viewer, the configuration can be somewhat awkward. As the viewer's head gets close to the transparency, for a more detailed look, it can obscure the front illumination over portions of the image. Also, the front illuminators must protrude significantly from the image which adds to the awkwardness of the viewing device. A simple portable view box becomes difficult to design.